dev: integration tests for multisig _mocks (proposal / signature-treasury / signature-mint-burn)

Context

#628 moved three multisig examples into contracts/test/integration/_mocks/ as
self-contained, deployable contracts — but added no fixtures or specs. The
other mocks there are already covered; these three are the gap.

Cover

• MultisigProposalTreasury — caller-authorized proposal governance over a shielded treasury (deposit, createShieldedProposal, approve/revoke, executeShieldedProposal).
• MultisigSignatureMintBurn — signature-authorized mint/burn of a native shielded token (3 signers, threshold 2).
• MultisigSignatureTreasury — signature-authorized single-tx execute from a stateless shielded treasury.

Task

• Wire each into compact:integration (contracts/package.json).
• Add fixtures (test/integration/fixtures/) + specs (test/integration/specs/).
• Cover happy paths and failure modes (non-signer, below-threshold, wrong color, replay).

ECDSA is stubbed in EcdsaSignerManager (#629), so signature paths can only assert membership/threshold until it lands. Overlaps #630 — keep one.

Relates to #628, #630.

Start with contracts/package.json and the existing fixtures and specs in test/integration/ to see how the other mocks are wired into compact:integration. Add coverage for MultisigProposalTreasury, MultisigSignatureMintBurn, and MultisigSignatureTreasury, including the listed happy and failure paths. Until EcdsaSignerManager is implemented, signature tests should assert membership and threshold behavior only.